| Product(s): | SewerGEMS, CivilStorm, OpenRoads |
| Version(s): | All |
| Area: | Modeling |
Problem
How do I model an infiltration trench that does not discharge water?
How do I model an infiltration trench where all the water is absorbed into the ground?
Solution - LID or Pond
There are different approaches that can be taken depending on your modeling needs. The two main options for modeling an infiltration trench are:
- Infiltration trench LID (low impact development) control
- Pond with seepage method
Low Impact Development Approach
One approach is to use the "infiltration trench" LID type. Lay out the LID element and parent catchment to represent the area of the trench, then select the catchment as the LID's parent catchment. For details on LIB/catchment connectivity, see: Modeling Low Impact Development (LID) Controls
With this, you can model the storage effect of your aggregate, connect upstream pipes that discharge to it, account for underdrain outflow, and account for overflow.
Model schematic:
In the LID Control properties
- Surface layer - define the properties of the surface area above the trench.
- Storage layer - define the properties of the trench fill (crushed stone aggregate, usually) and the ground infiltration rate.
- Underdrain layer - define the properties of the underdrain, which could be used to model a pipe in the trench which discharges some captured runoff to another location.
Considerations
- Catchment area: If the parent catchment represents a larger area that drains to the trench (and you don't have any pipes discharging into the trench), set the LID property "Occupies Full Catchment" to "False", then enter the area of the LID.
- Parent catchment outflow element - this can be directed to the same element as the underdrain outflow element, or a separate outflow node for example. Your infiltration trench will not have any outflow (unless using the underdrain), but in order for the model run there needs an outfall and a downstream element from the catchment.
- If pipes discharge into the trench - Use a headwall to model any pipes that discharge into the trench. In the headwall properties, set it to Free Outfall and choose the LID's parent catchment as the "Route to catchment". Ensure the LID is set to "occupy full catchment". The headwall discharge will then enter the LID along with any catchment rainfall runoff from the storm event. If you had been using a catchment larger than the size of the trench, use a new parent catchment with the same size as the LID (with "occupies full catchment" set to True), and then model another catchment for the area around it, with outflow element set to the LID's parent catchment.
- An alternative would be to run the model, copy the headwall/outfall hydrograph (Data tab of graph) and paste as an additional hydrograph inflow in the parent catchment properties. You would need to update it if the upstream pipe outflow changes.
- If trench overflow needs to be modeled - consider using the pond approach instead, using a composite outlet structure (such as a weir) to direct overflow to another location.
- To model a perforated pipe in the trench - consider using the LID underdrain option to model this (layer 4). In the LID element properties, choose an "underdrain outflow element" to direct the underdrain discharge to another location. For example use a transition element connected to a conduit.
Results interpretation
- Review summary mass balance in the "LID Performance Summary" of the Report section of the Calculation Summary. Note: using the latest available version is advised, as some earlier versions did not properly label the LIDs in this section of the report.
- Select "Detailed Results" for the "Output Options" in the properties of the LID, then you can also graph various results from the LID itself.
- Inflow - flow that enters the LID through the surface layer
- Surface infiltration - infiltration entering into the surface layer - will be equal to "Inflow" if all the surface layer inflow is from infiltration
- Surface Runoff - overflow that exits the LID off the surface layer - will be zero if it all soaks into the storage layer. It will be a positive value if the storage depth exceeds the surface storage physical depth. This overflow will be accounted for in the parent catchment's Outflow Element.
- Storage depth - depth of flow inside the storage layer
- Drain outflow - flow exiting through the underdrain
- Bottom infiltration - flow exiting out of the bottom and into the ground
- The water that soaks into the ground under the infiltration trench is accounted for in the "Bottom Infiltration" result and the water that leaves the optional underdrain (which could be a perforated pipe) is accounted for in the "drain outflow" and from graphing flow out of the element you selected for the "underdrain outflow element".
- Graphing flow for the parent catchment will not show infiltration.
Pond Approach
Ponds and pond outlets are another approach you can consider. This also enables you to model the storage effects, fill void space, infiltration (seepage), underdrain outflow and weir overflow. A schematic of typical connectivity is shown below:
Considerations:
- To account for void space from aggregate (stone fill) - use the void space column in the pond's elevation-area or elevation-volume curve to account for the fill.
- Note that pond void space is not currently supported when using the Explicit (SWMM) solver. If you need to use this solver, you will need to manually adjust the area or volume values for your pond to account for the fill void space. See: Using Pond Void Space
- To model infiltration (water soaking into the ground under the fill) - use one of the available pond seepage methods.
- Compatibility: Available Pond Seepage (infiltration) and evaporation methods per solver
- To view infiltration (seepage) results, graph the result "Pond (seepage loss)".
- If pipes discharge into the trench - connect it to the pond using an outfall or headwall. For an outflow, select "Boundary Element" as the Boundary Condition type and select the pond as the boundary element.
- To model underdrain flow - use a pond outlet structure configured as an orifice or user defined rating curve.
- To model overflow - use a separate pond outlet structure configured as a weir
Example Model
An example model is available at the following link, which demonstrates both of the above approaches. It is saved in version 26.00.00.514 so you may need to upgrade in order to open it, if you have an earlier version.
Infiltration Trench Modeling Approach Example Project